Every physical structure has a resonant frequency at which it vibrates when excited by a driving force. If the energy of the driving force is sufficiently great, sustained vibration of a structure at its resonant frequency may damage or destroy it. Even vibration at a relatively low amplitude may be undesirable in certain sensitive or critical components. For example, excitation of resonant vibration in the frame of a satellite optical system occurring each time that the device is moved and aimed at a different target may result in an unacceptably long time delay before the satellite optical system is again usable. In such components, it is important to quickly damp the vibration.
Conventional techniques to reduce the effect of vibration on a structure have used resilient materials, such as rubber, to isolate the structure from vibrations that would otherwise be induced through an attached support or mount. When applied along the surface of a component, elastic materials can also absorb energy directly from the component to reduce the duration of its vibration; however, in many applications, it is not practical to coat the surface of a structure with an elastic material or to use a resilient mount or suspension to connect the structure to a supporting body.
A passive method of vibration damping to control noise emissions is described in an article entitled, "The Quiet Alloys," published in Machine Design Magazine, Apr. 1978, pp. 202-206. This article suggests manufacturing components subject to vibration from metal alloys having a characteristic high specific damping capacity (SDC). Copper-manganese alloys (Incramute), nickel-titanium alloys (Nitinol), and other alloys of copper and nickel are proposed for such use, to damp vibration and reduce noise emanating from a component. The article also teaches that vibration can be reduced in a steel component by replacing about ten percent of its volume with plugs or rings comprising a high SDC metal alloy.
Active vibration damping has also been employed to reduce the amplitude of structural vibration, often much more rapidly than possible with passive, energy absorbing vibration damping materials. For example, in U.S. Pat. No. 4,724,923, an electromagnet is used to oppose the vibrational motion of a body, quickly restoring the body to its rest position. A control energizes the electromagnet in phase with the resonant frequency of the body so that a magnetic force is applied to restore the body to its rest position during each cycle of its vibratory motion, quickly damping the vibration. In many applications, it would be virtually impossible to mount an electromagnet adjacent a structure to damp its vibration. In addition, structures comprising only nonmagnetic materials would not be affected by the electromagnetic force.
Hydraulic or pneumatic actuators are also sometimes used in conventional active damping systems to provide a synchronous restoring force to damp structural vibration. However, the weight and size constraints of such devices significantly limit their usefulness.
Whether electromagnetic, hydraulic, or pneumatic, a damping actuator in an active vibration damping system should be controlled in response to the vibrational displacement of the effected structural component, so that the actuator provides a restoring force to oppose or resist vibrational motion, rather than driving the vibration to even greater amplitude. Accelerometers, strain gauges, LVDTs, and other types of conventional displacement, velocity, or acceleration sensors may be employed to provide a signal for controlling the application of a restoring force by an actuator used to damp the vibration of a structure. However, in structures subject to multimode vibration, e.g., vibration at multiple harmonics of the resonant frequency, a single such sensor is likely to be inadequate. Instead, a plurality of sensors (and a corresponding plurality of actuators to provide the restoring force) should be disposed at a number of spaced apart positions within the structure to sense and damp each node of its multimode vibration. Unfortunately, for many applications, use of conventional displacement/motion vibration sensors and conventional hydraulic or pneumatic damping actuators to control multimode vibration would probably be prohibitively expensive and complex.
There are other practical limitations to conventional active vibration damping systems. Attachment of vibration sensors and conventional active damping actuators to a structural component may interfere with its function. For example, the aerodynamic properties of a helicopter rotor blade would likely be adversely affected by externally mounted hydraulic or pneumatic dampers. Mounting conventional active damping actuators internally within a structure could significantly weaken it due to the size of such devices. In fact, virtually none of the prior art active vibration damping systems are sufficiently compact to be integrated within most structural components.